Modding OEM Downpipe.

[NORCAL SS]

bump

[shagghie]

How's it goin up there NorCal??? Any love yet on the 2.5" mod?

[NORCAL SS]

LOVING my 2.5 cat back. not sure for this weekend. I have headers to do on 2 customers cts-vs

[shagghie]

LOVING my 2.5 cat back. not sure for this weekend. I have headers to do on 2 customers cts-vs

Right on... still thinking you need a resonator?

[NORCAL SS]

no not at all. Got used to it and its just a tad louder than the road race axle back. my 64 year old mother in law drives the car also from time to time has no problem with it. best of all no problem with the ride

[dylansi]

Tony,

I dont think the exit diameter being as small as it is, is holding back flow. I think the cat is the biggie here. From my own personal story i will reflect here, haha.

Catless DP - felt slow as heck, only reason I feel this is true is because you are losing that surge of boost that the stock cat or any other cat holds. With the staright through muffler, after losing the cat, there isnt anything hold back flow, so the pressure is not resticted and power feels like an N/A car. It may have made more power but def didnt feel like it.

DP with high flow cat - boost surge is back BUT not as much as stock. You still have that "push you back into your seat" rush so the cat is def holding back flow.

Stock DP - stomp on throttle, wait for it, wait for it, wait for it, whoosh, now boost. Cat is holding back exhaust until it reaches the point of surging through the cells.

So from that experience I dont think that flange being increased to 2.5" is really going to make a difference. The cat is the culprit.

What is needed is a high flow metallic cat (heats up faster than ceramic and should eliminate CEL p0420) around 200 cell, welded in right after a catless DP of 2.5" ID, that will still accomidate the secondary o2 sensors location and be smog legal. From there 2.5" ID pipe to a single exit, either in the center or the drivers side rear valance.

If that setup could be mass produced you would see a great deal of business and I am sure great gains in top end power.

[Abarth Five O]

This is good feedback from Dylansi who installed and tested both the TMC decat DP and R/R HF cat DP on his Abarth and is sharing from his real life experience which I can attest to.

[NORCAL SS]

All good. I do a lot of cars like to test out my theory

my 2.5 cat back didnt kill power at all on my car. I now beat my escalade on the freeway which mid to high 14s consistant, where as before the 2.5 id didn't really inch away at all.

[dylansi]

All good. I do a lot of cars like to test out my theory

my 2.5 cat back didnt kill power at all on my car. I now beat my escalade on the freeway which mid to high 14s consistant, where as before the 2.5 id didn't really inch away at all.

I believe you will or have seen a small gain and applaud your persistance in trying to figure this exhaust out but I just wanted to give you my .02, so you are not wasting your time.

Pretty much remove restrictive cat, replace with a high flow unit, increase pipe diameter throughout the hole setup to 2.5" and your done.

Find out where the other vendors are getting there flanges from, take your car to a buddies shop, have them weld on those flanges to a 2.5" ID pipe running turbo down add in a high flow cat, continue with 2.5" ID single pipe back. Problem solved...

There are just so many exhaust threads here with people trying to figure out something that isnt that difficult. Only thing difficult is finding out what size pipe will net the highest gains before increasing the size becomes counter-productive.

[musicsurf]

I believe you will or have seen a small gain and applaud your persistance in trying to figure this exhaust out but I just wanted to give you my .02, so you are not wasting your time.

Pretty much remove restrictive cat, replace with a high flow unit, increase pipe diameter throughout the hole setup to 2.5" and your done.

Find out where the other vendors are getting there flanges from, take your car to a buddies shop, have them weld on those flanges to a 2.5" ID pipe running turbo down add in a high flow cat, continue with 2.5" ID single pipe back. Problem solved...

There are just so many exhaust threads here with people trying to figure out something that isnt that difficult. Only thing difficult is finding out what size pipe will net the highest gains before increasing the size becomes counter-productive.

Turbo cars do not work like NA cars when it comes to back pressure. The compressor turbine is driven by the exhaust gases. The faster the gases are able to exit, the faster the turbine can spin and the faster the turbo can spool. No cat = less restriction and a faster spooling turbo. That's why boost creep can be an issue (maybe not with our cars) with a catless dp. Now, if there was a boost leak at the dp/turbo then the reading would be screwed on the first o2 sensor and could cause problems. I don't think with the stock cat exhaust gases build up and then flow through. It's like pouring water in a cup with a small hole, the water is only going to flow as fast as the hole allows it to. That also applies to the 1 7/8" exit in the stock dp. I have no idea if the stock cat is more of a restriction than the exit diameter, I would think it's not judging by NorcalSS's impression of changing the exhaust and eliminating the restriction in the midpipe while opening up the exhaust.